The present invention relates to control of a thermal head of a thermal printer, and more particularly, to a circuit for controlling the area modulation of a transfer pixel unit.
FIG. 9 illustrates a conventional circuit for controlling the energizing of heating elements. In FIG. 9, numeral 101 denotes heating elements, 108 transistors for driving the respective heating elements, 107 AND gates, 106 latches, 104 a shift register, 109 an energizing time signal line, and 102 a printing data line through which one-bit printing data is input on a time series basis, the printing data being transferred to the shift register 104 in synchronization with a clock signal from a transfer clock line 103. After all the printing data for turning on or off the heating elements 101 have been transferred, a latch signal from a latch signal line 105 causes the printing data stored in the shift register 104 to be collectively stored in the respective latches 106. Before being converted into a signal for driving the transistor 108, the output of the latch 106 is input to one terminal of AND gate 107, while a signal from the energizing time signal line 109 is input to the other terminal of AND gate 107. The heating element 101 is energized via the transistor 108 by making the printing data "1" (HIGH), while the heating element is energized only for the period of time defined by the energizing time signal after the printing data is completely stored in the latch 106. With this arrangement, the energizing time can be varied by supplying to each heating element the number of pulses corresponding to image density, where 256 dots in terms of a binary data image is equivalent to one line. For example, the energizing time can be varied with respect to a maximum dot of one pixel in a multi-value data image when tone is emphasized, by individually controlling the heating-element energizing time as in the case of a multi-value data image, or when energizing time control at a level of 256 is exercised on the individual heating element.
As described above, the energizing of one heating unit is determined by the time required for data to be transferred to the register in view of the prior art circuit configuration. If it is attempted to control the energizing time to increase the image density, the printing speed per line tends to decrease. In other words, simultaneous control of printing speed and energizing time has heretofore been incompatible. In order to solve this problem, there has been proposed a method of controlling the energizing time by transferring energizing time data to a multi-bit shift register and inputting the data to a counter with a latch function to allow individual control of the energizing times of the heating elements. Although simultaneous control of printing speed and energizing time has been made compatible in the method described above, the center of a dot shifts in proportion to its diameter (.DELTA.L), as shown in the transfer pixel configuration of FIG. 10. The disadvantage in this case is that the dot corresponding to the pixel tends to be inclined, although at a small angle.